Use charset UTF-8 in Custom Json Handler

[TsimurSh]

Unfortunately these changes "Use UTF-8 as character encoding for XML responses" #376 - don't help. You can also test it by using GTFS with polish characters: https://www.ztm.poznan.pl/pl/dla-deweloperow/getGTFSFile/?file=20240924_20240930.zip

[CLAassistant]

All committers have signed the CLA.

[aaronbrethorst]

Please reopen after my questions/requests are addressed. thanks!